That one is not optimized. I get hundreds of fps with libsvtav1, give it a try

were you trying the default av1 encoder in ffmpeg? that one is unoptimized try libsvtav1 I get hundreds of fps, albeit on a 9800x3d

not a wise choice but one nonetheless

use software transcoding if thats your issue

if plex cannot work at all with AV1, it might be time to move to a non-garbage media server like jellyfin.